Coat a 9×13-inch pan with cooking spray; set aside. Melt butter in a large saucepan over low heat. Add marshmallows and stir until mixture is completely smooth. Remove from heat and add 15-25 drops of red food coloring to reach desired color. Add Rice Krispies and stir until coated.
Spread mixture into prepared pan. Use a rubber spatula to press mixture so that it is distributed evenly. Let pan sit for 1 hour. Using heart-shaped cookie cutters, cut out shapes and set hearts on a cutting board.
Enclose chocolate in a Ziploc bag* and drop bag into a bowl of hot tap water; let sit for 5 minutes. Massage with fingers until smooth. Cut a ¼-inch piece off of a corner of the bag and spread over the tops of the treats. Smooth the tops with the back of a spoon, letting chocolate drip down sides. Sprinkle décor over top, if desired. Let chocolate cool and serve. *If desired, divide chocolate between multiple bags and (once melted) add food coloring. Massage coloring into chocolate.